1. Field of the Invention
The present invention relates to a high-voltage power source apparatus and, more particularly, to a high-voltage power source apparatus for use in a copying machine, a printer, a television, or the like.
2. Description of the Prior Art
As one type of conventional high-voltage power source apparatuses, a circuit as shown in FIG. 1 is frequently used in controlling a charger in a copying machine.
The circuit shown in FIG. 1 generates a high-voltage current obtained by superposing a desired AC voltage on a high DC voltage. A rectangular wave (or sine wave) pulse is supplied from terminals 3 and 4 to a step-up transformer 6. Then, an AC voltage stepped up in accordance with the turn ratio is induced in the secondary winding. The AC voltage is rectified and smoothed into a high DC voltage V1 by a rectifier consisting of a diode 8, a capacitor 9 and a load resistor 10.
An AC voltage of a frequency determined in accordance with the control characteristics of a desired load is applied to a transformer 5 from terminals 1 and 2. The high DC voltage from the transformer 6 is applied to one end of the secondary windinq of the transformer 5. Therefore, a voltage (V1+V2) is generated at the secondary winding of the transformer 5. The voltage is obtained by superposing the DC voltage V1 and an AC voltage V2 obtained by stepping up the AC voltage supplied from the terminals 1 and 2 (the voltages V1 and V2 are shown in FIG. 2). The voltage (V1+V2) is supplied to a load through an output terminal 7.
As shown in FIG. 2, the output voltage is a voltage which is obtained by superposing an AC voltage stepped up by the transformer 5 to the voltage V1 stepped up by the transformer 6.
The above-described conventional circuit for generating an AC voltage requires two transformers for the DC and AC voltages. Therefore, the circuit is expensive. Furthermore, since transformers are relatively large in size, the overall circuit becomes large in size. The DC voltage V1 as shown in FIG. 2 is frequently high, e.g., several kV. This results in a considerably high cost for insulation between the primary and secondary windings of the transformer 5 and for insulation between the high- and low-voltage portions. When a superposing AC voltage of a low frequency is to be obtained in order to obtain desired load control characteristics, the transformer 5 must be rendered large in size in order to prevent magnetic saturation of the transformer 5. This also results in making the overall circuit heavy.